Silicone compositions are well known products offering premium properties in a wide variety of applications. For example, certain flexible silicone resin systems have been found useful as electronic junction coatings which are used for passivating and protecting semi-conductor surfaces, and serve to assure optimal electrical performance. Such silicone materials are chosen since they offer rapid cure rates which improve production efficiency. The low durometer of these materials helps to minimize the vibration and shock experienced by such semiconductor devices and the low viscosity of such silicone coating compositions provides for ease of handling.
Such silicone electronic junction coatings are useful in a number of applications. A flexible conformal coating of such material can provide mechanical and electrical insulation prior to molding of plastic power transistors, for example. These materials can also be used as a light pipe or lens which insulates and isolates LED's and photocouplers and may be utilized with or without further encapsulation. These silicone junction coating materials are also a resiliant coating which can be used directly over transistors, diodes and monolithic integrated circuit chips in order to prevent damage during and after final potting or sealing. Additionally, these coatings can be used as a thin section potting material for extra protection in cavity type packages in order to provide primary mechanical and chemical protection.
Heretofore, silicone electronic junction coating materials have ordinarily been two part or two package systems which when mixed provide a platinum catalyzed addition cure reaction system. Ordinarily the first package will contain a major amount of a vinyl functional linear siloxane fluid having a specified viscosity as will be described below, and an effective amount of a precious metal or platinum catalyst, as is well known in the art. Generally the second package will contain an additional amount of the vinyl functional siloxane fluid material in combination with a methyl hydgrogen siloxy functional silicone fluid. This methyl hydrogen siloxane functional fluid is ordinarily known as the crosslinking agent since it provides the hydrogen which adds to the doubly bonded vinyl group in the platinum catalyzed addition reaction in order to provide a tightly crosslinked and cured final product. Although the coating blend of parts 1 and 2 is ordinarily cured at elevated temperatures, such as, approximately 200.degree. C., the two package system is nonetheless necessary since the blended material will begin to gel even at room temperature, and can become useless in as little as 24 hours. Therefore, there has been a widespread need for inhibitor compositions which will be effective for delaying the addition cure reaction for varied periods of time depending upon the uses of the material. For example, an inhibitor can be useful even though a two part system is utilized in order to provide a useful, workable pot life for the catalyzed material after it is blended. Additionally, a search has been made for an effective inhibitor which would obviate the need for a two package system since it would be effective for inhibiting even the catalyzed blend of the previously discussed two part system. That is to say, it would be especially preferable if a one package electronic junction coating could be provided which contained an inhibitor composition which allowed for a sufficiently long shelf life yet would cure to an effective junction coating upon the application of an elevated temperature. One of the purposes of the present disclosure is to demonstrate that a particular combination of inhibitors has been found to be effective for such purposes when used together, and provide better results when either of the types of inhibitors is used separately.
Triallylisocyanurate has been utilized previously as an inhibitor in certain addition cure silicone systems. See, for example, U.S. Pat. No. 3,882,083 of Berger et al. which is hereby incorporated by reference which utilizes an ethylenically unsaturated isocyanurate inhibiting composition in a platinum catalyzed, room temperature vulcanizable (RTV) silicone composition. It has been found, however, that for the electronic junction coating compositions of the present invention the use of triallylisocyanurate alone provides an insufficiently long pot life, i.e. it becomes unworkable in too short a period of time.
On the other hand, dialkylacetylenedicarboxylates have been suggested as inhibitors in other types of platinum catalyzed silicone compositions such as the silicone release coatings of R. P. Eckberg disclosed in the copending application Ser. No. 250,479 filed Apr. 2, 1981, and which is hereby incorporated by reference. But, again it has been found that the use of such dialkylacetylenedicarboxylate inhibitors alone provides for unsatisfactory slow cure of such electronic junction coatings while at the same time being relatively expensive materials, and additionally requiring too great an increase in the silicone hydride crosslinking component in order to provide adequate cure of these coatings.
Additionally, not only has it been found that the dual inhibitor system described above provides excellent inhibiting properties in a platinum catalyzed addition cure electronic junction coating system, it has further been discovered that this dual system provides surprising adhesion promotion properties heretofore unavailable in such addition cure silicone systems. Certain heretofore known platinum catalyzed addition cure materials have had notoriously poor adhesion to substrates, and often adhesion promotors have been utilized to alleviate this deficiency. However, it has now been discovered that the combination of triallylisocyanurate and dialkylacetylenedicarboxylate not only provides effective inhibiting properties, but additionally provides for tenacous adherence of the silicone material to a coated substrate. Thus this improved adhesion is believed to be a fortuitous result of the search for effective inhibitors for such electronic junction coatings.
Thus it is an object of the present invention to provide one package silicone electronic junction coating compositions.
It is another object to provide a dual inhibitor system effective for prolonging the shelf life and work like of such junction coating compositions.
It is another object to provide junction coating compositions having improved adhesion to substrates.
These and other objects will become apparent to one skilled in the art upon consideration of the present specification and claims.